This is the largest family of the mosses in number of genera. Its taxonomy is commonly considered difficult because of the obscure areolation, small size of the plants, and apparent phenotypic variation. A recent generic-level revision by R. H. Zander (1993) pulled together the scattered literature and de-emphasized sporophytic characters, allowing easier identification of sterile plants. The phylogenetic scheme adopted here approximates that of Zander (1993) as modified by Zander (2006) except that Gymnostomiella and Luisierella are moved from the Barbuloideae to the Pottioideae, in part for convenience in identification. The large number of anatomical characters available permits better identification of previously poorly understood taxa. The color tests refer to the reaction of the cell walls of the distal laminal areolation to 2% potassium hydroxide solution, which may require examination under the compound microscope for determination. Sectioning of leaf and stem is necessary and is described by Zander (1993). A twisted peristome, strongly differentiated costal anatomy, and the complexly papillose distal laminal cells are characteristic of this mostly acrocarpous family, commonly found in harsh environments.

Leaves usually broadly ligulate to spathulate, with one stereid band in the costa; clavate gemmae rare; stem sclerodermis commonly not or poorly differentiated, cells of central cylinder intergrade in size with cortical cells